1. Field of the Invention
This invention relates to a liquid crystal apparatus and typically to a liquid crystal display apparatus for use as a display unit, and more particularly to a liquid crystal display apparatus which operates in a light scattering mode.
2. Description of the Related Art
Various liquid crystal modes are available such as a TN (Twisted Nematic) mode, a VA (vertical alignment) mode which uses vertical orientation and an IPS (In-Plane-Switching) mode and an FFS (Fringe Field Switching) mode wherein switching is carried out within a substrate plane. Scattering type liquid crystal is a mode wherein switching is carried out between a light scattering state and a no-light scattering state or transmission state. Research and development of such scattering type liquid crystal are proceeding energetically because the scattering type liquid crystal generally requires no polarizing plate and can achieve brighter display than the aforementioned liquid crystal modes which essentially require a polarizing plate.
Generally, high molecular dispersion type liquid crystal is used as the scattering types liquid crystal. A principle of the high molecular dispersion type liquid crystal is described with reference to FIGS. 1A and 1B. The high molecular dispersion type liquid crystal apparatus includes a pair of glass substrates 1, a pair of ITO (Indium Tin Oxide) electrodes 2, polymer 3, and liquid crystal molecules 4. In fabrication, liquid crystal to which monomer is added is injected into a cell which has electrodes at upper and lower portions thereof, and then ultraviolet rays are irradiated to cause phase separation of the liquid crystal and the high molecules. Thereupon, the liquid crystal is placed into small balls, which are surrounded by the polymerized high molecules.
In a state wherein no voltage is applied, the liquid crystal molecules in the small balls are directed at random and the average refractive index of the liquid crystal is different from the high molecules around the small balls. Therefore, light is scattered by the liquid crystal as shown in FIG. 1A. On the other hand, when a voltage is applied, where the liquid crystal has the positive dielectric anisotropy, the major axis of the liquid crystal is oriented so as to be parallel to the electric field E as shown in FIG. 1B. At this time, if the liquid crystal is made of a material with which the refractive index of the liquid crystal in the minor axis direction is substantially equal to the refractive index of the surrounding high molecules, then the light is not scattered by but is transmitted through the liquid crystal. This is the principle of the high molecule scattering type liquid crystal.
Also scattering type liquid crystal which uses dynamic scattering is available. In the scattering type liquid crystal just described, liquid crystal generally having negative dielectric anisotropy and ions are injected in an element having electrodes at upper and lower portions thereof. If a high voltage is applied to the element described, then turbulent flows are produced in the liquid crystal, and light is scattered by the liquid crystal (dynamic scattering).
A liquid crystal display apparatus in the past frequently adopt a vertical electric field system wherein a voltage is applied in a thicknesswise direction or vertical direction of the liquid crystal. In contrast, in recent years, attention is attracted to a liquid crystal display apparatus which adopts the transverse electric field method. This is the IPS mode or the FFS mode wherein switching is carried out within a plane of a substrate and is disclosed, for example, in Japanese Patent Laid-Open No. 2005-338264. The liquid crystal display apparatus of the transverse electric field type includes a pair of substrates, liquid crystal held in a gap between the substrates, electrodes formed on one of the substrates, and driving means for applying a transverse electric field along the planar direction of the substrate to the liquid crystal through the electrodes. In the liquid crystal display apparatus, the orientation state of the liquid crystal is controlled through the transverse electric field to carry out display. In comparison with the liquid crystal display apparatus of the vertical electric field type, the liquid crystal display apparatus of the transverse electric field type is characterized in that it has a large angular field of view and exhibits a high contrast and attracts attention as a display unit for a portable apparatus.
The transverse electric field mode is described with reference to a plan view of FIG. 2A and a sectional view of FIG. 2B. In the transverse electric field mode, a common electrode 6 is provided commonly on a driving substrate 5, and a pixel electrode 8 is formed in a comb-shaped pattern on the common electrode 6 with an insulating film 7 interposed therebetween, and an orientation film not shown is provided in such a manner as to cover over the pixel electrode 8. A liquid crystal layer 10 is held between the orientation film on the driving substrate 5 side and another orientation film not shown of an opposing substrate 9. A pair of polarizing plates 11 and 11a are disposed in a crossed Nicol relationship with the substrates interposed therebetween. Here, the rubbing direction of the orientation film on the driving substrate side and the rubbing direction of the orientation film on the opposing substrate side, that is, the orientation direction of the liquid crystal, coincides with the transmission axis of one of the two polarizing plates. Further, where the dielectric anisotropy of the liquid crystal layer is positive, the rubbing direction and the extending direction of the pixel electrode are substantially parallel to each other. Actually, the rubbing direction and the extending direction of the pixel electrode define therebetween an angle θ such that the direction in which the liquid crystal rotates when a voltage is applied thereto may be controlled.
In the transverse electric field mode having such a configuration as described above, when no voltage is applied between the common electrode and the pixel electrode, the orientation direction of the liquid crystal molecules which form the liquid crystal layer is perpendicular to the transmission axis of one of the polarizing plate but parallel to the transmission axis of the other polarizing plate. Consequently, the pixel displays the black. On the other hand, if a voltage is applied between the common electrode and the pixel electrode, then the orientation direction of the liquid crystal molecules is rotated to an oblique direction with respect to the extending direction of the pixel electrode by a transverse electric field or fringe field generated between the pixel electrodes. Consequently, light transmitted through the liquid crystal layer is provided with a rotated polarization state and therefore is transmitted through the opposing side polarizing plate, resulting in display of the white. The transverse electric field mode described above is called fringe field switching mode or FFS mode. As another one of transverse electric field modes, for example, an IPS mode is known.